1. Field of the Invention
The present invention relates to an ink jet printer of an on-demand type and, more particularly, to a technique to prevent a recording head from being clogged.
2. Prior Art
An ink jet recording head of an on-demand type includes a plurality of nozzle openings and a plurality of pressure generation chambers respectively in communication with the nozzle openings, and is arranged such that, responsive to a printing signal, it expands or contracts the pressure generation chambers to generate ink droplets.
When the ink droplets attach to a recording medium, they can run on it, depending on to the quality thereof or they can contact with other printer members to get rubbed. For this reason, the ink droplets are prepared such that the solvent will evaporate to solidify the ink as soon as possible. Due to this, when a printing operation is interrupted or in the case of a nozzle opening through which the ink droplets are not frequently discharged, the solvent will evaporate to thereby cause the nozzle opening to be clogged with the ink.
To solve such problem, when the printing operation is to be stopped for relatively many hours, it is necessary to mount a cap onto the nozzle opening to thereby prevent the ink solvent from evaporating. However, even during the printing operation, all of the nozzle openings do not generate the ink droplets equally and, according to the positions where the nozzle openings are arranged, there can exist some nozzle openings where the frequency of ink jet generation is very low.
To solve such problem, there is proposed a method in which, when a printing operation has been performed continuously for a given time, a recording head is stepped aside into a non-printing area and then ink droplets are forcibly jetted out from all of the nozzle openings. However, this method required interruption of the printing operation, which results in the lowered printing speed.
To solve this problem, there is also proposed a clogging preventive technique in which a print signal is applied through a current limit resistance to a piezoelectric vibrator disposed in a pressure generation chamber in communication with a nozzle opening through which no ink droplet will be generated during the printing operation, thereby vibrating slightly a meniscus in the neighborhood of the nozzle opening (see Unexamined Japanese Patent Publication No. Sho. 55-123476, Unexamined Japanese Patent Publication No. Sho. 57-61576, U.S. Pat. No. 4,350,989).
According to the clogging preventive technique, because the need to interrupt the printing operation is eliminated, the clogging of the nozzle opening can be prevented without lowering the printing speed. However, to change the amplitude for the slight vibration, a supply voltage, a resistance value and the like must be adjusted, so that circuit the resulting configuration becomes complicated.